Multiple input multiple output (MIMO) systems are wireless communications systems that may transmit signals utilizing a plurality of transmitting antennas, and/or receive signals utilizing a plurality of receiving antennas. Communications between MIMO systems may be based on specifications from the Institute of Electrical and Electronics Engineers (IEEE). A MIMO system that receives a signal Y may compute a channel estimate matrix, H, based on the received signal. The signal may comprise information generated from a plurality of information sources. Each such information source may be referred to as a spatial stream.
An exemplary wireless communication system is a wireless local area network (WLAN) system, such as is defined by IEEE 802.11 specifications. In a WLAN a physical layer protocol data unit (PPDU) represents the basic data unit transmitted and/or received in a WLAN system. The PPDU may, in turn, comprise a series of fields, such as a data field. The data field within a PPDU may also be referred to as a physical layer service data unit (PSDU), or payload. An exemplary PPDU definition may be found in an applicable standards document, such as IEEE 802.11.
A MIMO transmitter may combine spatial streams to generate one or more to-be-transmitted RF chains. A group of to-be-transmitted RF chains may be simultaneously transmitted as a signal vector X. A transmitting MIMO system may utilize a plurality of transmitting antennas when transmitting the signal vector X. A receiving MIMO system may utilize a plurality of receiving antennas when receiving a corresponding signal vector Y.
When the number of to-be-transmitted RF chains is less than the number of transmitting antennas at the MIMO transmitter, an RF chain may be transmitted simultaneously via a plurality of transmitter antennas. This technique may be referred to as transmitter diversity. When utilizing transmitter diversity, the MIMO transmitter may assign, or map, individual to-be-transmitted RF chains for transmission via individual transmitting antennas. Similarly, a MIMO receiver may utilize receiver diversity when the number of received RF chains is less than the number of receiving antennas. When utilizing receiver diversity, the MIMO receiver may assign, or map, individual received RF chains to individual receiving antennas.
Antenna selection is a procedure which may enable a MIMO transmitter to map individual RF chains to individual transmitting antennas, and/or enable a MIMO receiver to map individual RF chains to individual receiving antennas. In conventional MIMO systems, antenna selection procedures at the MIMO transmitter may enable a transmitting antenna to transmit a sequence of sounding frames that enable a MIMO receiver to communicate antenna selection feedback information. Based on the antenna selection feedback information, the MIMO transmitter may enable mapping of individual to-be-transmitted RF chains to individual transmitting antennas at the MIMO transmitter.
Under some circumstances, the antenna selection procedure may end before completion. For example, the MIMO receiver may detect a missing sounding frame in the sequence of sounding frames transmitted by the MIMO transmitter, or the MIMO transmitter may receive old, or “stale”, feedback information from the MIMO receiver. In instances when the antenna selection procedure ends before completion, the MIMO transmitter and MIMO receiver may need to restart the antenna selection procedure from the beginning. In such case, the MIMO transmitter may restart the antenna selection procedure by retransmitting at least the first sounding frame in the sounding frame sequence.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.